1. Field of the Invention
The present invention relates to the field of radiofrequency transmitters, especially those used in wireless communication terminals such as mobile telephones for example.
The invention relates, more especially, to reconfigurable radiofrequency transmitters which are capable of transmitting over a wide power range and using several transmission formats.
2. Description of Related Art
In the field of wireless radio communication, there are numerous communication formats such as the UHF, GSM, EDGE, WCDMA, 3G, etc. standards. Each of these standards is associated with a particular transmission frequency band (one talks of the UHF band, GSM band, 3G band, etc. for example).
As radio frequency transmissions are being deregulated, there are plans to adopt the concept of opportunistic use of the radiofrequency spectrum. The 4G standard, for example, takes up this concept. According to this opportunistic concept, a wide spectrum of radiofrequencies is scanned (several hundred kilohertz to several tens of megahertz) and bands which are not occupied by frequencies are detected. Radio transmission then takes place in an unused frequency band in accordance with a corresponding communication standard.
This type of opportunistic communication therefore assumes that the transmitter of the wireless communication terminal has a high degree of reconfigurability so that it can transmit using several transmission formats and in several frequency bands. The transmitter must also switch rapidly from one type of transmission to another. In fact there is a risk that the frequency band which was detected as being unused will no longer be unused if switchover takes too long.
Nevertheless, a radio transmitter which is suitable for transmitting in accordance with a particular standard is not generally suitable for transmitting according to another standard. For example, the architecture of a radio transmitter according to the GMSK standard is based on direct modulation and, to achieve this, comprises an analogue modulator followed by a power amplifier. In contrast, polar loop architectures are especially suitable for transmitting non-constant envelope signals such as the 8PSK modulation adopted for the EDGE standard. But the polar loop cannot transmit signals with a bandwidth larger than EDGE signals. On the other hand, only an architecture with direct modulation and a linear amplifier is usually feasible for wideband transmissions such as transmission according to the 3G and OFMD standards.
Documents U.S. Pat. No. 6,339,621 and EP 1 662 665 disclose radio transmitters which use digital modulation. In these transmitters, quadrature digital signals are initially each modulated by a sigma-delta filter, then the output signals from the filters are combined by using quadrature modulation. A modulated radiofrequency digital signal is obtained in this way.
In the transmitters proposed in these documents, the operating frequency, which is the transmission frequency, is fixed so that it is suitable for a particular application. It is not possible to obtain opportunistic communication using such systems. Even if the input quadrature digital signals of the sigma-delta filters were programmed in order to meet a selected communication standard depending on the available frequency band, obtaining a digital radiofrequency signal modulated at the correct transmission frequency would still pose problems. In fact, the transmission frequency is of the order of several gigahertz for certain types of communications. This is where the components of the transmitters reach their operating limits.
There are terminals equipped with several transmitters, each of which is dedicated to a particular standard, in order to overcome the incompatibilities between various radiofrequency transmission architectures. In order to cover the entire radiofrequency spectrum, it is therefore necessary to provide a considerable number of transmitters dedicated to the standards associated with the various frequency bands in the radiofrequency spectrum. Such terminals are therefore bulky and poorly optimised. In addition, the speed of changeover from one type of transmission to another type of transmission is limited, making use of a terminal of this type for opportunistic use of radiofrequency spectrum is therefore difficult to envisage.